Add some style guide verbiage

STYLEGUIDE.md

@@ -4,21 +4,51 @@
 ### Organization
 ### Naming
 
+* Use hyphens instead of underscores in file and directory names
+
 ## Formatting
 
+* Code must meet the requirements of our options to `clang-format`
+
 ## Naming Conventions
 
 ## Comments and Documentation
 
 ## Header Guidelines
 
+* Headers should follow [include-what-you-use](https://include-what-you-use.org/) guidelines
+* Use `#pragma once` instead of include guards
+* Place local headers (quoted) ahead of system headers (brackets), with each block in alphabetical order
+* Public headers MUST have Doxygen markup for all public symbols
+* Private headers should have Doxygen markup flagged with `@internal`
+
 ## Language Features and Idioms
-### C++
-### C
+
+### CMake
+* We use CMake 3.x (see the root `CMakeLists.txt` for the specific version) and have not moved to 4.x yet
+* Use modern CMake (e.g., 3.x) paradigms and avoid "legacy CMake"
+
+### C/C++
+
+* C++17 and C11 are supported
+* Use modern C++ idioms
+* Do not use GNU extensions
+* Code should be platform-independent
+
+### Shell scripts
+
+* All shell scripts should be run through [ShellCheck](https://www.shellcheck.net/) and report no issues
+* Use `#!/usr/bin/env bash` as a platform-independent shebang line
+* bash-isms are allowed, within reason
 
 ## Error Handling
 
 ## Testing Conventions
 
+* All new functionality introduced MUST include tests of the feature
+    * This could include unit tests, system tests, and integration tests
+    * These tests MUST be automatable
+* Bugfixes MUST include a proof-of-concept test that fails before the fix and passes after
+
 ## Tooling and Enforcement
 clang-tidy, formatter, etc.